1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of thin film transistors (hereinafter, referred to as TFTs) and a method of manufacturing the semiconductor device. For example, the present invention relates to an electro-optical device typified by a liquid crystal display panel and an electronic equipment mounted with the electro-optical device as a component.
Note that the term semiconductor device in this specification indicates devices in general capable of functioning with the use of semiconductor characteristics, and electro-optical devices, light emitting devices provided with EL elements and the like, semiconductor circuits and electronic equipment are all included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, a technique of constituting a thin film transistor (TFT) by using a semiconductor thin film (with a thickness of approximately several to several hundred of nm) formed on a substrate having an insulating surface has attracted attention. The thin film transistor is widely applied to an electronic device such as an IC or an electro-optical device, and needs to be developed promptly as, in particular, a switching element of an image display device.
An active matrix liquid crystal module, an EL module, and a contact image sensor are known as typical examples of the thin film transistors. Particularly, a TFT having a silicon film having a crystalline structure (typically, polysilicon film) as an active layer (hereafter, referred to as polysilicon TFT) has high filed effect mobility, and thus a circuit with various functions can be formed by using the TFT.
For example, in a liquid crystal module mounted to a liquid crystal display device, a pixel portion for performing image display for each functional block and a driver circuit for controlling the pixel portion, such as a shift register circuit, a level shifter circuit, a buffer circuit or a sampling circuit, which is based on a CMOS circuit are formed on one substrate.
Further, TFTs (pixel TFTs) are respectively arranged in several tens to several million of pixels in the pixel portion of the active matrix liquid crystal module, and pixel electrodes are provided to the respective pixel TFTs. Opposing electrodes are provided in an opposing substrate sandwiching liquid crystal with a substrate, and a sort of capacitor with the liquid crystal as dielectric is formed. A voltage applied to the respective pixels is controlled with a switching function of the TFT to control charge to the capacitor to thereby drive the liquid crystal. Thus, a light transmission amount is controlled, thereby displaying an image.
The pixel TFT consists of an n-channel TFT, and applies a voltage to the liquid crystal to drive it as a switching element. Since the liquid crystal is driven by an alternating current, a system called frame inversion driving is often adopted. In this system, in order to suppress power consumption at a low level, it is important to sufficiently lower an off-current value (drain current that flows at the time of off-operation of the TFT) for a characteristic required for the pixel TFT.
Further, in order to manufacture a TFT having superior electrical characteristics at lower cost, a laser annealing technique that enables processing for a short period of time has been essential.
Laser annealing is generally used for a process of crystallizing an amorphous semiconductor film, a process of improving crystallinity, and the like. Note that a laser often used for laser annealing is an excimer laser. A method of conducting laser annealing in which: a laser beam emitted from a pulse oscillation laser with large output is processed by an optical system so as to have a shape of a square spot several by several centimeters or a linear shape with a length of, for example, 10 cm or more on an irradiation surface; and an irradiation position of the laser beam is scanned relative to the irradiation surface, is preferably used since the method provides high productivity and is superior in mass-production. Particularly, when a laser beam having a linear shape (hereinafter referred to as linear beam) is used on the irradiation surface, differently from the case where a spot laser beam, which needs scanning in back and forth directions and right and left directions, is used, the laser beam can be irradiated over the irradiation surface only with the scanning in a direction perpendicular to a line direction of the linear beam, which provides high productivity. The reason the scanning is performed in the direction perpendicular to the line direction is that the perpendicular direction is the most effective scanning direction. Due to the high productivity, the use of the linear beam from a large-output laser, which is processed by an appropriate optical system, is becoming the main stream in laser annealing. Further, the linear beam is irradiated in an overlapping manner while gradually shifting in a short direction, whereby laser annealing is conducted to the entire surface of an amorphous silicon film to crystallize the film or improve the crystallinity.
Further, in order to manufacture a TFT at lower cost, it has been essential to manufacture the TFT on a glass substrate which is cheaper than a semiconductor substrate or a quartz substrate and which can attain a large surface area thereof.
In case of using the glass substrate, in order to prevent alkaline metal contained in the glass substrate from diffusing, a base insulating film comprised of an insulating film containing silicon as its main constituent (silicon oxide film, silicon nitride film, silicon oxynitride film, or the like) is provided, an amorphous silicon film is formed on the film, and then, laser light irradiation is conducted.
The present inventors found a large number of minute holes in the surface of the silicon film that has undergone laser irradiation through many experiments and studies. The minute hole is very small, and a photograph of the hole in SEM (magnification of 35 thousand) observation is shown in FIG. 26. The present inventors found that variation is caused among a large number of TFTs formed on a substrate with the cause of unevenness of the surface of a semiconductor film due to the minute hole. In the case where the active layer of the TFT is formed at the position of the minute hole, the TFT has the poor electrical characteristics in comparison with other TFTs manufactured on the same substrate.
Further, the minute hole often occurs in the case where laser light is irradiated with a relatively high energy density or a relatively high overlap ratio. In particular, there is a tendency that the minute hole appears remarkably in the case where laser light is irradiated in a nitrogen atmosphere or a vacuum.
Moreover, the minute hole occurs in the case where the amorphous silicon film is formed on the base insulating film, but does not occur in the case where the amorphous silicon film is formed contacting the substrate without forming the base insulating film.
Based on the above, the present inventors made many experiments and studies from various angles in order to pinpoint the cause of occurrence of the minute hole. As a result, they further found that minute convex portions were formed in the surface of the amorphous silicon film before laser light irradiation. This minute convex portion is also very small (typically, with a diameter of 1 xcexcm or less and a height of 0.05 xcexcm or less), and a photograph of the convex portion in SEM (magnification of 50 thousand) observation is shown in FIG. 25. Note that when the minute convex portion and the vicinity thereof are measured by EDX analysis, it is confirmed that the convex portion is not impurities such as dust.
When the minute convex portion is irradiated with laser light, the minute hole is easy to occur. The present inventors found that the minute convex portion is the cause of occurrence of the minute hole.
The minute convex portion is formed at the step of forming the amorphous silicon film on the base insulating film, and can be observed as an extremely small luminous point by microscopic observation in a dark-field reflection mode with magnification of 500.
Means can be adopted in which the base insulating film is not formed. However, the base insulating film is provided in order that impurity ions such as alkaline metal contained in the glass substrate do not diffuse into a semiconductor film formed above the base insulating film, and is indispensable for manufacture of the TFT at lower cost.
The present invention has been made in view of the above, and an object of the present invention is therefore to form a base insulating film and an amorphous semiconductor film in lamination on an inexpensive substrate (glass substrate or the like) and to suppress occurrence of a minute convex portion and of a minute hole due to the convex portion even with laser light irradiation. That is to say, an object of the present invention is to obtain an amorphous semiconductor film having an excellent surface in flatness on a base insulating film.
In order to solve the above-mentioned problems, many experiments and studies were made from various angles. As a result, a film deposition temperature of the base insulating film and a film deposition temperature of the amorphous semiconductor film are made substantially equal to each other, whereby the amorphous semiconductor film having a surface which does not have the minute convex portion and which is excellent in flatness can be obtained. Thus, the occurrence of the minute hole can be suppressed even with laser light irradiation.
Note that xe2x80x9cfilm deposition temperatures are made substantially equal to each otherxe2x80x9dindicates that the ratio of the film deposition temperature of the amorphous semiconductor film to the film deposition temperature of the base insulating film is 0.8 to 1.2, preferably that the difference between the base insulating film and the amorphous semiconductor film in film deposition temperature is in a range of xc2x150xc2x0 C.
According to a first structure of the present invention disclosed in this specification, there is provided a method of manufacturing a semiconductor device, comprising:
a first step of forming a base insulating film on an insulating surface;
a second step of forming an amorphous semiconductor film on the base insulating film; and
a third step of performing crystallization by irradiation of laser light to the amorphous semiconductor film, thereby forming a semiconductor film having a crystalline structure,
characterized in that a film deposition temperature of the base insulating film is the same as a film deposition temperature of the amorphous semiconductor film.
According to a second structure of the present invention, there is provided a method of manufacturing a semiconductor device, comprising:
a first step of forming a base insulating film on an insulating surface;
a second step of forming an amorphous semiconductor film on the base insulating film; and
a third step of performing crystallization by irradiation of laser light to the amorphous semiconductor film, thereby forming a semiconductor film having a crystalline structure,
characterized in that a difference in film deposition temperature between the base insulting film and the amorphous semiconductor film is in a range of xc2x150xc2x0 C.
The film deposition temperature of the base insulating film and the film deposition temperature of the amorphous semiconductor film are made substantially the same, whereby the semiconductor film surface with high flatness can be obtained. By using the semiconductor film with high flatness in the active layer of the TFT, the withstand voltage is raised. Thus, the reliability of the TFT is improved.
Further, the present invention is applicable to not only laser light irradiation in crystallization but also a process with laser light used in the manufacturing process of the semiconductor device, for example, laser annealing used for the improvement of film quality and for the activation of the impurity element.
According to a third structure of the present invention, there is provided a method of manufacturing a semiconductor device, comprising:
a first step of forming a base insulating film on an insulating surface;
a second step of forming an amorphous semiconductor film on the base insulating film; and
a third step of performing laser light irradiation to the amorphous semiconductor film,
characterized in that a difference in film deposition temperature between the base insulting film and the amorphous semiconductor film is in a range of xc2x150xc2x0 C.
Further, in prior art, as a leveling process, there are given an etchback method, in which etching is performed to attain leveling after the formation of an application film, a mechanical chemical polishing (CMP) method, and the like. However, in the present invention, it is only necessary that the film deposition temperature is made the same between the base insulating film and the amorphous semiconductor film, and the reduction in the film thickness due to leveling and the increase in the number of steps are not effected.
Further, the present invention is particularly effective in the case where the base insulating film is required as in case of the glass substrate.
According to a fourth structure of the present invention, there is provided a method of manufacturing a semiconductor device, comprising:
a first step of forming a base insulating film on an insulating surface;
a second step of forming an amorphous semiconductor film on the base insulating film;
a third step of performing crystallization by irradiation of laser light to the amorphous semiconductor film, thereby forming a semiconductor film having a crystalline structure and an oxide film on the semiconductor film,
a fourth step of removing the oxide film; and
a fifth step of performing laser light irradiation in an inert gas atmosphere or in a vacuum, thereby leveling the surface of the semiconductor film,
characterized in that a difference in film deposition temperature between the base insulting film and the amorphous semiconductor film is in a range of xc2x150xc2x0 C.
Further, the present invention is particularly effective since minute holes are easy to occur in the case where laser light is irradiated to the semiconductor film in a vacuum or in an inert gas atmosphere.
Further, in the fourth structure, it is characterized in that energy density of the laser light in the fifth step is higher than energy density of the laser light in the third step.
Further, in the fourth structure, it is characterized in that an overlap ratio of the laser light in the fifth step is lower than an overlap ratio of the laser light in the third step.
According to a fifth structure of the present invention, there is provided a method of manufacturing a semiconductor device, comprising:
a first step of forming a base insulating film on an insulating surface;
a second step of forming an amorphous semiconductor film on the base insulating film;
a third step of doping the amorphous semiconductor film with a metal element;
a fourth step of performing heat treatment to the semiconductor film and then performing laser light irradiation, thereby forming a semiconductor film having a crystalline structure and an oxide film on the semiconductor film;
a fifth step of removing the oxide film; and
a sixth step of performing laser light irradiation in an inert gas atmosphere or in a vacuum, thereby leveling the surface of the semiconductor film,
characterized in that a difference in film deposition temperature between the base insulting film and the amorphous semiconductor film is in a range of xc2x150xc2x0 C.
According to a sixth structure of the present invention, there is provided a method of manufacturing a semiconductor device, comprising:
a first step of forming a base insulating film on an insulating surface;
a second step of forming a first semiconductor film having an amorphous structure on the base insulating film;
a third step of doping the first semiconductor film having an amorphous structure with a metal element;
a fourth step of performing heat treatment to the first semiconductor film and then performing laser light irradiation, thereby forming a first semiconductor film having a crystalline structure and an oxide film on the first semiconductor film;
a fifth step of removing the oxide film;
a sixth step of performing laser light irradiation in an inert gas atmosphere or in a vacuum, thereby leveling the surface of the first semiconductor film;
a seventh step of oxidizing the surface of the semiconductor film having a crystalline structure with a solution containing ozone, thereby forming a barrier layer;
an eighth step of forming a second semiconductor film containing a noble gas element on the barrier layer;
a ninth step of gettering the metal element to the second semiconductor film, thereby removing or reducing the metal element in the first semiconductor film having a crystalline structure; and
a tenth step of removing the second semiconductor film and the barrier layer,
characterized in that a difference in film deposition temperature between the base insulting film and the first semiconductor film having an amorphous structure is in a range of xc2x150xc2x0 C.
Further, in the sixth structure, it is characterized in that the noble gas element is one or a plurality of elements selected from the group consisting of He, Ne, Ar, Kr and Xe.
Further, in the sixth structure, it is characterized in that the second semiconductor film is formed by sputtering with semiconductor as a target in an atmosphere containing the noble gas element.
Further, in the fifth structure or the sixth structure, it is characterized in that the heat treatment in the fourth step is a heating process or a process of irradiating a strong light. The strong light is a light emitted from one selected from the group consisting of a halogen lamp, a metal halide lamp, a xenon-arc lamp, a carbon-arc lamp, a high-pressure sodium lamp and a high-pressure mercury lamp.
Further, in the fifth structure or the sixth structure, the metal element is one or a plurality of elements selected from the group consisting of Fe, Ni, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au, which are elements that accelerate crystallization of silicon.
Further, in each of the above-described structures, the laser light is emitted from an excimer laser, an Ar laser or a Kr laser of continuous oscillation or pulse oscillation type, or a YAG laser, a YVO4 laser, a YLF laser, a YAlO3 laser, a glass laser, a ruby laser, an alexandrite laser, or a Ti:sapphire laser of continuous oscillation or pulse oscillation type.
Further, in the fourth structure, the fifth structure or the sixth structure, the inert gas atmosphere is a nitrogen atmosphere.
Further, in the fourth structure, the fifth structure or the sixth structure, the second laser light irradiation is a leveling process performed in a vacuum or in an inert gas atmosphere, and the surface of the semiconductor film is further leveled. Particularly in the case where the gate insulating film is thin, for example, the gate insulating film has a thickness of 100 nm or less, the present invention is very effective.